1. Field of the Invention
The present invention relates to a communication system, a transmitting apparatus and a transmitting method, a receiving apparatus and a receiving method, an unbalance code mixing (UCM) method and a multiple code decoding method, which are operating under a multiple access environment where a plurality of mobile terminals are in communication with a single base station simultaneously, and more particularly, to a communication system, a transmitting apparatus and a transmitting method, a receiving apparatus and a receiving method, an unbalance code mixing and a multiple code decoding method, which are to expand a capacity by canceling in- and out-cell interference.
More specifically, the present invention relates to a communication system, a transmitting apparatus and a transmitting method, a receiving apparatus and a receiving method, an unbalance code mixing method and a multiple code decoding method, which are to increase a capacity by performing operations with a very short frequency reuse, more particularly, to a communication system, a transmitting apparatus and a transmitting method, a receiving apparatus and a receiving method, an unbalance code mixing method and a multiple code decoding method, which are to increase a capacity by implementing one-frequency reuse using a non-spread spectrum cellular system.
2. Description of the Related Art
The cause of mobile communications was because electromagnetic waves were discovered, and thereafter, further researches and developments of the mobile communications have been made due to the necessity for communication with ships, airplanes and/or trains. Further, the subject of the mobile communications has been widened to cover a range of communication with automobiles and/or humans. The mobile communications have also become adaptable not only to transmission of data with telegraphy and/or telephones but also to transmission of computer data and multimedia contents such as images.
In recent years, size and cost reductions of a mobile terminal are rapidly being improved with increasing fabrication technologies and the like. Also, the mobile terminal is increasingly being personalized like a mobile phone with expanding information communication services and the like. Further, mobile terminal users are being on an increase more and more as communication liberalization, communication charge reductions and the like become attainable.
Fundamentals of mobile communications are that a mobile station such as an automobile phone and a mobile phone effects detection of its nearest base station to exchange radio waves between the mobile station and the base station. A serviceable range of communication adaptable to reception of the radio waves from one base station is called a cell. The cell is typically in the shape of a circle having a given radius with a base station antenna as a center. Then, a communication service area is configured in such a way as to dispose the cells closely.
FIG. 19 schematically illustrates a cell configuration in a mobile wireless communication system such as a cellular system that permits planar development of a service area with a plurality of base stations. A broad service area is configured with the plurality of base stations (not shown) spaced at certain intervals to continuously lay a plurality of cells distributed by each base station as shown in FIG. 19.
A reason why the mobile communication system is employing the cells as described above is to provide the advantages of being able to effectively use a limited frequency resource by limiting a range of reception of the radio waves from the base stations to within a specific cell to ensure that a reuse of the same frequency is executable in other cells, and of achieving a size reduction and a power saving of a mobile terminal typically packaged as a battery-driven portable device by adapting segmentation into cells to reduction of output of the radio waves for communication. In recent years, factors such as an increased number of mobile phone users (cellular phone users) have increasingly required a technology for accepting a large number of mobile phone users as much as possible to a cell, while maximizing the effective utilization of the limited frequency resource. A single cell permits therein the existence of a plurality of mobile terminals, which are in communication with the single base station simultaneously. Thus, as judging from a base station side, a multiple access, in other words, a technology for multiplexing a radio signal to detect which signal is assigned to which user or multi-user detection is required.
A Frequency Division Multiple Access (FDMA), a Time Division Multiple Access (TDMA) for use in a second generation of a Personal Digital Cellular (PDC) and a Code Division Multiple Access (CDMA) for use in a third generation of the PDC have been conventionally known in Japan as multiple access technologies in wireless communications.
The TDMA is a system, which is to carry out communication by assigning a different time slot for each mobile terminal for simultaneous communication under the condition that a communication channel is divided into sections with the time slot on a temporal axis in advance. This communication system is premised to be of digital system. In a digital mobile phone system in Japan, time-division multiplexing into three or six channels takes place.
The FDMA is a system, which is to carry out communication by assigning a different frequency to each channel between mobile terminals for simultaneous communication or for each speech channel. Specifically, the FDMA is to use an available channel by allowing appropriate assignment thereof with a large number of communication channels arranged on a frequency axis. The FDMA is adaptable to any of analog and digital communication systems. In Japan, the FDMA is in use for automobile phones and mobile phones of an analog system.
The CDMA is a system, which is to carry out communication by applying spectrum spreading to share a wide range of frequencies with a plurality of mobile terminals. Whenever the mobile terminals are in communication, a spreading sequence for spectrum spreading is assigned to each mobile terminal, which then transmits a communication signal after spreading of the communication signal with the spreading sequence. The CDMA allows the mobile terminals to use a common frequency, so that all communication signals of stations other than one's own station result in interference to one's own station, and a performance of extracting a received signal out of the interference affects greatly on a reception level.
Now, a communication capacity is defined as the number of users acceptable to one channel for one cell. The most serious problem awaiting solution under the wireless communication environment such as that the coexistence of a large number of mobile stations within the single cell brings about because of rapid and wide spread of the mobile communications is how an increased capacity is attained with a limited resource.
The TDMA and the FDMA perform a reuse of a plurality of frequencies by assigning different frequencies to adjacent cells. A capacity of each of these systems is dependent only upon the number of channels. By contrast, the CDMA uses the same frequency between and within the cells simultaneously, and thereby suffers in- and out-cell interference. Specifically, the CDMA is considered to be a system whose capacity is not dependent upon the number of channels but upon an amount of interference.
The FDMA and the TDMA determine the upper limit of the number of users acceptable to one cell with reference to the number of channels obtained by segmenting a serviceable frequency bandwidth, and are thus limited in capacity. Also, the FDMA and the TDMA are incapable of executing the reuse of the same frequency among the cells adjoining to one another, and are thus considered to be of small capacity as the whole of communication services.
The CDMA effects code division using the spreading sequence consisting of orthogonal and pseudo-orthogonal codes, while users acceptable to one cell share the same frequency, so that all signals for other users result in undesired interference waves. The spreading sequence applied to each mobile terminal is recognizable at a base station side, so that the base station may detect each user signal. Conversely, the spreading sequence applied to other mobile terminals is not recognizable at a mobile terminal side, so that user detection is not executable. While it may be good if the spreading sequence is completely in an orthogonal arrangement, components other than orthogonal components are attributable to interference components, with the result that the number of users acceptable to one cell is smaller in proportion to the number of channels made up of the pseudo-orthogonal codes. Also, the CDMA has a tendency to use a broad frequency bandwidth because of a need for spreading, and is thus considered to be of small capacity although one-frequency reuse is executable.
The communication system based on the CDMA may achieve detection of each signal, specifically, multi-user detection by applying an interference cancellation technology such as an interference canceller IC (See Reference Document of Patent 1, for instance). The interference canceller IC is effective in detecting all received signals by repeating a process of demodulating, in order of the magnitude of reception power, the received signals that are composed of the sum of noise and in-coming signals having made propagation from each transmitting-side station through each propagation characteristic, and of canceling one's own signal.
When inter-cell multi-user detection is effected with the interference cancellation technology such as the interference canceller IC, a receiving-side station detects the in-coming signal from each of in- and out-cell transmitting-side stations on the assumption that the in-coming signal is a desired signal, so that the adjacent or neighboring cells themselves may share the channel of the same frequency spatially or temporally. Thus, a multi-cell configuration based on one-frequency reuse is realizable with the TDMA and/or the FDMA and thus makes contribution toward more efficient utilization of the frequency and also toward an increased capacity as in a case of the frequency of the same utilization.
However, a difference in reception power between a signal interfered with other cells and the desired signal is supposed to be made smaller in the vicinity of a cell boundary, leading to a case where reception of the above signals with equal power occurs. Under the above conditions, a problem of failing to achieve decoding and cancellation with the interference canceller IC arises.    [Reference Document of Patent 1]    Japanese Patent Laid-open No. 2002-84214